Two novel applications of ultrasonic waves to tissue characterization are described in this proposal: 1) In the first, high frequency acoustic waves scatter off in vitro biological cells in suspension as they are individually convected through the focal zones of confocally positioned transducer. When scattering data are combined with data on the cell volume, obtained from an in-line, electrozone sensing element, one can in principle deduce the volume, compressibility, and density of each cell, and thus determining information on the distribution of properties for a heterogeneous cell population. This mechanical characterization of cell properties may offer new dimensions by which to detect and/or assess disease state or pathologies, and may be adaptable to cell sorting or usable in conjunction with laser cell flow cytometry apparatus for more complete cell characterization. 2) In the second application, new, precise experimental data on the nonlinear parameter of materials now offer the possibility of determining the composition of tissues. Initial tests of a new theoretical methodology suggest that percentages of water, protein, and fat in tissue samples can be determined from ultrasonic measurements. In performing precise ultrasonic measurements and testing (and, where appropriate, modifying) the theory, points of contact will be sought with a) the extensive data base on the acoustic absorption in tissues, protein solutions, and lipid suspensions and b) the recent results of those groups seeking to use the nonlinear acoustic property as an imaging parameter. In the latter case, if spatial information obtained from imaging could be combined with the tissue composition methodology, in vivo spatial information on tissue composition might be possible. The proposed work points in the direction of this important goal.